Hot melt adhesives are versatile in industrial applications such as bonding articles, for example, cardboard boxes, plastics parts, bookbinding and furniture manufacturing. Hot melt adhesives generally comprise 100% solids, and, in commercial applications, are typically used at temperatures at about 350 degrees F. These materials do not generate volatile compounds during the task of bonding, which is particularly advantageous in high-speed packaging operations where there is little time for drying an adhesive that contains solvent. The lack of volatile emissions is also an advantage over adhesives containing organic solvents, because the emission organic solvents, many of which are known to be toxic and may have an adverse effect when released into the environment, may pose environmental and health concerns.
Hot melt adhesive compositions are typically thermoplastic materials that are heated to a molten state for their application to cardboard or corrugated cartons, boxes, plastics and other items that need to be sealed or to be adhered to. For example. after application of the molten adhesive to a substrate such as paper (or similar cellulosic fibrous article), a second item that needs to be bonded to the substrate is placed in contact with the molten hot melt adhesive. As the molten adhesive cools and solidifies, a bond is formed between the substrate and the second item that has utility in many areas of commerce including consumer and industrial packaging.
The time a hot melt adhesive takes to solidify to a point where it can no longer bond with the intended article is called the ‘open time’.
The time required for the adhesive to cool to the point where it has enough strength to form a bond is the ‘set speed’. Set speed is an important parameter for applications such as high speed packaging lines, where bonding needs to occur rapidly to avoid poorly sealed or unsealed boxes from exiting the packaging line and having to be rejected, for example, in applications where items such as consumer packaged goods are being sealed.
Hot melt adhesives are generally composed of three components: (1) a polymer resin; (2) a tackifier; and (3) a wax. The polymer provides the formulation with its strength and adhesive characteristics. The polymer in the formulations generally range from 25 to 50% by weight of the formulation.
The tackifier allows the polymer to be more adhesive by improving wetting during the application. Tackifying agents are added to give tack to the adhesive and also to lower viscosity. Tack is required in most adhesive formulations to allow for proper joining of articles prior to the hot melt adhesive solidifying.
The wax reduces the overall viscosity of the adhesive, thereby allowing it to liquefy. The wax also controls the open time and set speed of the system. In the present invention, the use of a wax having a low but relatively sharp melting point leads to a hot melt adhesive composition which liquefies at a temperature lower than that of commercially available hot melt adhesives, but still retains good set speed characteristics.
The ratio of polymer to tackifier to wax is generally specific to a particular application's need. In general, the percent wax is minimized and added in sufficient quantities to achieve desired viscosity and set speeds. The level of wax is generally in the range of 15 to 35% by weight of the formulation. The waxes employed in most hot melt adhesive compositions are waxes which are derived from petroleum, such as the paraffins and microcrystalline waxes.
Depending upon the specific application desired, various other components, such as plasticizers, are used in the formulation of hot melt adhesives. Plasticizers reduce the overall viscosity of the adhesive and promote flexibility and wetting. Typical types of plasticizers used are phthalates, glycolates, polybutenes, and mineral oil. See “Adhesive Bonding”, Chap. 8 Hot-melt Adhesives, by Thomas Flanagan, at p. 8-3, published by MacMillan.
In formulating adhesives all the ingredients should be compatible, so the formulation does not separate in the molten storage tank. Separation would result in the inability to apply the adhesive and also poor adhesive performance. The closer the solubility parameters of the different components are to each other, the greater the compatibility. See “Some Factors Affecting the Solubility of Polymers”, by P. A. Small, J. Appl. Chem., 3, February 1953 (which teaches how to predict solubility parameters).
The cloud point temperature is the temperature at which a component begins to solidify or “cloud up” as it cools from a clear liquid phase to the solid phase. For example, for waxes, the cloud point is usually close to the melting point of the wax. Compatibility is related to cloud point temperature, where generally, the lower the cloud point temperature, the greater the compatibility. See “Adhesives and Coatings Manual” by National Distillers and Chemical Corporation (1983).
Most hot melt adhesive formulations utilize polymer resins having a low vinyl acetate content due to its lower cost relative to high vinyl acetate content polymers. Low vinyl acetate polymers are relatively non polar and can be formulated with other relatively non-polar tackifiers and waxes to yield compatible formulations. The commonly used waxes are non polar and can be formulated into compatible adhesives with low vinyl acetate containing polymer; these waxes include various grades of paraffin wax.
Higher vinyl acetate content polymer resins (>18% Vinyl acetate) are also used in hot melt adhesive formulations. The higher vinyl acetate content results in a stronger ionic bond to polar substrates such as paper, thereby creating a stronger adhesive. Vinyl groups are also known to modify polymer physical properties, making the composition more pliable and thereby increasing its adhesion performance, as known to those skilled in the art. The use of higher vinyl acetate content polymers requires formulating using more polar waxes and tackifiers to maintain formulation compatibility. These more polar waxes are generally more expensive than paraffin wax and the selection and supply of these more polar waxes is limited. Fischer-Tropsch (“FT”) waxes are often used with higher vinyl acetate content polymer due to their better compatibility. From a supply standpoint however, Fischer-Tropsch waxes are not produced in the Americas. The two largest suppliers of FT waxes are Sasol from South Africa and Shell Oil from Malaysia; supplies of FT waxes are thus potentially subject to supply interruptions caused by world events.
Large oil companies such as Shell Oil, ExxonMobil and other oil refiners supply petroleum waxes used in these applications. Most of this wax is derived in the process of refining lube oil where the wax is separated from the lube oil stock and refined into various fractions of wax including paraffins, and microcrystalline waxes. Formulators such as Astor Wax, IGI and Moore & Munger also supply wax for these applications that is resold as is from the oil companies, and/or formulated and repackaged to meet the specific needs of customers.
A wax that is to be used in hot melt adhesives must have a relatively sharp melt point to yield an adhesive with a short ‘set speed’ and controllable open time. The melt point is another property in addition to compatibility. The wax must also allow for a reduction of overall adhesive viscosity to allow for the proper application or coating of the hot melt adhesive on the intended substrate. Generally, hot melt adhesive formulations are heated to 300-350 degrees F. prior to application in order to reduce viscosity. The wax must be stable at these temperatures to allow for extended periods as a molten product prior to application. It is well known to those versed in the art that stabilizers such as antioxidant (for example, hindered phenols) and free radical scavenger (such as, but not limited to, butylated hydroxy toluene “BHT”, butylated hydroxyanisole “BHA”, and Irganox 1010, supplied by Ciba Corp.) compounds can be added to the adhesive compound to further enhance thermal stability.
Synthetic ethylene vinyl acetate (“EVA”) waxes have been developed and are commercially available for use with high vinyl acetate content polymer in adhesive formulations. Low molecular weight ethylene vinyl acetate waxes such as AC 400 (available from Honeywell); EVA1 (BASF); and MC400, available through Marcus Oil and Chemical, are examples of such commercially available materials. These waxes, however, are not widely used because of their relatively high cost to manufacture and resulting high selling price. These waxes also have relatively poor set speed characteristics when incorporated into adhesive formulations due to their low crystallinity and a lack of a sharp melting point.
Various attempts to utilize alternatives to imported and/or petroleum derived waxes have been reported (U.S. Pat. No. 4,749,739, U.S. Pat. No. 4,396,673 and U.S. Pat. No. 4,388,138; these patents are incorporated by reference herein).
For example, Foster, et al. (U.S. Pat. No. 4,749,739) discloses incorporation of a synthetic polyethylene wax, hydrocarbon tackifier and amorphous propylene polymer to create a low viscosity hot melt adhesive.
Ball, et al. (U.S. Pat. Nos. 4,396,673 and 4,388,138) mentions use of vegetable wax in combination with an isocyanate binder as a release agent in the manufacture of particleboard.
Mehaffy, et al (U.S. Pat. No. 6,117,945) highlights the need for low application temperature (between 200 to 300 degrees F.) hot melt adhesives and suggests a styrene, alpha-methylstyrene and/or vinyltoluene polymer combined with ethylene vinyl acetate polymer and paraffin wax.
The prior art thus illustrates the use of petroleum-derived waxes and synthetic waxes for formulating hot melt adhesive compounds. There are no mentions of vegetable derived triglyceride waxes for use in hot melt adhesive formulations, yet there is a recognized and long-felt need to find alternatives to products such as petroleum waxes that are derived from scarce and limited natural resources. There is also a recognized and long-felt need to use materials in hot melt adhesives that are considered safe to humans because of the adhesives' use in the manufacture of containers used to transport and store foodstuffs. There is also a recognized and long-felt need to use materials in hot melt adhesives that are naturally derived and can be easily recycled back into the environment without long-term adverse effects; corrugated cartons having wax-based coatings and adhesives, for example, are known to be difficult to recycle. Therefore, there is a need for employing a wax, which has similar properties of petroleum derived or synthetic waxes used in hot melt adhesive formulations. Due the large volume of waxes consumed in these applications it is also preferred that the compositions be readily available. From both a supply and a natural resource viewpoint, it is preferred that the compositions be obtained from a source that preferably is renewable, such as from plant extracts.
There is a need for a wax that is compatible with high vinyl acetate content polymer resins, has a sharp melting point, low viscosity, does not adversely affect adhesion and is thermally stable. It is also desirable to have a wax that does not have to be imported, and produced at a cost which is competitive with that of the paraffin and microcrystalline waxes. Given that the world's petroleum supply is finite, and dwindling, it is also desirable to have a wax that can be obtained from a renewable source, such as plants, rather than being petroleum based. Further, because hot melt adhesives are frequently used in food packaging applications, it is also desirable for the wax to have food grade properties for safety. The waxes of the present invention meet the rigorous requirements for this and other applications.
The present invention is a natural wax for use in hot melt adhesive formulations. The product is a commercially available high triglyceride wax derived from the processing of natural oil containing commodities such as soybeans, palm and other crops from which oil can be obtained. Hydrogenated vegetable oils are widely used in the food industry. Products of the present invention are highly hydrogenated to minimize un-saturation. Although commercially available, these materials are not widely produced or used due to their limited applications in the food industries. The materials are processed and supplied by Archer Daniels Midland (Decatur Ill.) designated by their product number 86-197-0, Cargill Incorporated (Wayzata, Minn.) designated by their product number 800mrcs0000u and other sources under a generic name ‘hydrogenated soybean oil’. Palm oil wax was supplied by Custom Shortenings & Oils (Richmond, Va.) and was designated as their product Master Chef Stable Flake-P.